Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of the substrate gas in a liquid aqueous menstruum with microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, propanol and n-butanol. The production of these oxygenated organic compounds requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, from the reforming of natural gas and/or biogas from anaerobic digestion or from off-gas streams of various industrial methods. The gas substrate contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. (For purposes herein, all gas compositions are reported on a dry basis unless otherwise stated or clear from the context.)
These substrate gases are typically more expensive than equivalent heat content amounts of fossil fuels. Hence, a desire exists to use these gases efficiently to make higher value products. The financial viability of any conversion process, especially to commodity chemicals such as ethanol and acetic acid, will be dependent upon capital costs, the efficiency of conversion of the carbon monoxide and hydrogen to the sought products and the energy costs to effect the conversion.
Syngas fermentation processes suffer from the poor solubility of the gas substrate, i.e., carbon dioxide and hydrogen, in the liquid phase of the aqueous menstruum where the biological processes occur. Munasinghe, et al., in Biomass-derived Syngas Fermentation in Biofuels: Opportunities and Challenges, Biosource Technology, 101 (2010) 5013-5022, summarize volumetric mass transfer coefficients to fermentation media that are reported in the literature for syngas and carbon monoxide in various reactor configurations and hydrodynamic conditions. A number of conditions can enhance the mass transfer of syngas to the liquid phase. For instance, increasing the interfacial area between the gas feed and the liquid phase can improve mass transfer rates.
Numerous processes have been disclosed for the conversion of carbon monoxide and hydrogen to oxygenated compounds. One such process suspends the microorganisms for the conversion in an aqueous menstruum contained in a stirred tank reactor such as by using a motor driven impeller. Stirred tank fermentation reactors provide many advantages. For stirred tank reactors, increasing the agitation of the impeller improves mass transfer as smaller bubble sizes are obtained. Also, the stirring action not only distributes the gas phase in the aqueous menstruum but also the duration of the contact between the phases can be controlled. Another very significant benefit is that the composition within the stirred tank can be relatively uniform. For instance, Munasignhe, et al., in a later published paper, Syngas Fermentation to Biofuel: Evaluation of Carbon Monoxide Mass Transfer Coefficient (kLa) in Different Reactor Configurations, Biotechol. Prog., 2010, Vol. 26, No. 6, pp 1616-1621, combine a sparger (0.5 millimeter diameter pores) with mechanical mixing at various rotational rates to provide enhanced mass transfer. This uniformity enables good control of the fermentation process during steady-state operation. This is of particular advantage in the anaerobic conversion of carbon monoxide and hydrogen to oxygenated compounds where two conversion pathways exist. Hence the carbon dioxide generated from the conversion of carbon monoxide is proximate in location to the hydrogen consumption pathway that consumes carbon dioxide. The uniformity further facilitates the addition of fresh gas substrate. The problems with stirred tank reactors are capital costs, the significant amount of energy consumed in the needed mixing and agitation, and the need for plural stages to achieve high conversion of substrate.
Another type of fermentation apparatus is a bubble column reactor wherein the substrate gas is introduced at the bottom of the vessel and bubbles through the aqueous menstruum (“bubble reactor”). See Munasinghe, et al., in Biomass-derived Syngas Fermentation in Biofuels: Opportunities and Challenges, Biosource Technology, 101 (2010) 5013-5022. In order to achieve sought mass transfer from the gas to liquid phases, workers have provided the gas feed to bubble columns in the form of microbubbles. Microbubble spargers were used to generate microbubbles. The authors report that in one study, the mass transfer obtained for a bubble column reactor was higher than that for a stirred tank reactor mainly due to the higher interfacial surface area obtained because of the smaller bubble size generated by the sparger used with the bubble column reactor.
Bredwell, et al., in Reactor Design Issues for Synthesis-Gas Fermentations, Biotechnol. Prog., 15 (1999) 834-844, assessed various types of reactors including bubble columns and stirred reactors. The authors disclose using microbubble sparging with mechanical agitation. At page 839 they state:                “When microbubble sparging is used, only enough power must be applied to the reactor to provide adequate liquid mixing. Thus axial flow impellers designed to have low shear and a high pumping capacity would be suitable when microbubbles are used in stirred tanks.”They conclude by stating:        “An improved ability to predict and control coalescence rates is needed to rationally design commercial-scale bioreactors that employ microbubble sparging.” (p. 841)        
Advantageously, commercial-scale bubble column reactors are relatively simple in design and construction and require relatively little energy to operate. However, microbubble spargers, especially for very small microbubbles, use significant amounts of energy and are prone to fouling. Accordingly, other means for generating microbubbles such as injectors using a motive fluid that are not prone to fouling, are preferred. Co-pending U.S. patent application Ser. No. 12/826,991, filed on Jun. 30, 2010, herein incorporated by reference in its entirety, discloses the use of injectors to supply gas feed to an anaerobic fermentation in a deep reactor to make a liquid product such as ethanol wherein the presence of the liquid product enables the injector to produce a dispersion of microbubbles.
For deep, bubble column reactors, the height of the aqueous menstruum is a primary determinant of the gas/liquid contact time. This height also is a determinant of the static head at the bottom portion of the reactor. Higher pressures result in smaller bubble sizes and higher partial pressures both of which enhance mass transfer efficiency and gas substrate conversion efficiency in the fermentation reactor. Thus, on a commercial scale, deep, bubble column reactors would have to have a depth of at least about 10, preferably at least about 15, meters and use microbubbles of gas feed in order to achieve viable conversion efficiencies. However, without the mechanical stifling of a stirred tank reactor, the compositions of the gas phase and liquid phase in a deep, bubble column reactor change over the depth of the vessel due to bioconversions and changes in gas solubility in the aqueous phase.
In their earlier review article, Munasignhe, et al., report that the gas-liquid mass transfer is the major resistance for gaseous substrate diffusion. The authors state at page 5017:                “High pressure operation improves the solubility of the gas in the aqueous phase. However, at higher concentrations of gaseous substrates, especially CO, anaerobic microorganisms are inhibited.”        
Other workers have understood that the presence of excess carbon monoxide can adversely affect the microorganisms and their performance. See paragraphs 0075 through 0077 and 0085 though 0086 of United States published patent application No. 20030211585 (Gaddy, et al.) disclosing a continuously stirred tank bioreactor for the production of ethanol from microbial fermentation. At paragraph 0077, Gaddy, et al., state:                “The presence of excess CO unfortunately also results in poor H2 conversion, which may not be economically favorable. The consequence of extended operation under substrate inhibition is poor H2 uptake. This eventually causes cell lysis and necessary restarting of the reactor. Where this method has an unintended result of CO substrate inhibition (the presence of too much CO for the available cells) during the initial growth of the culture or thereafter, the gas feed rate and/or agitation rate is reduced until the substrate inhibition is relieved.”        
At paragraph 0085, Gaddy, et al., discuss supplying excess carbon monoxide and hydrogen. They state:                “A slight excess of CO and H2 is achieved by attaining steady operation and then gradually increasing the gas feed rate and/or agitation rate (10% or less increments) until the CO and H2 conversions just start to decline.”        
The gas feed is introduced at the bottom of a deep, bubble column reactor where the most favorable conditions for mass transfer of carbon monoxide from the gas to the liquid phase exist. Hence, to avoid carbon monoxide inhibition, operating parameters such as carbon monoxide mole fraction in the gas feed, gas feed supply rate and microbubble size must be controlled to assure that the carbon monoxide mass transfer rate does not become so great as to cause carbon monoxide inhibition. However, the conditions required to avoid carbon monoxide inhibition in a deep, bubble column reactor negatively affect the overall amount of carbon monoxide that can be transferred to the liquid phase, and thus the conversion efficiency to oxygenated organic compound.
This negative effect is exacerbated since the static pressure is reduced as the microbubbles pass upwardly, the partial pressure of carbon monoxide in the bubbles decreases and the surface area to volume of the microbubbles may decrease. Furthermore, the compositions of the gas and liquid phases change over the height of the aqueous menstruum, further negatively affecting mass transfer of hydrogen and carbon monoxide to the liquid phase. Carbon dioxide co-product is generated by the carbon monoxide pathway and the solubility of carbon dioxide in the aqueous menstruum is highly sensitive to pressure. Thus, at higher elevations of the aqueous menstruum, carbon dioxide can pass to the bubbles and reduce the mole fractions of hydrogen and carbon monoxide in the gas phase. The reduced mole fractions also reduce the driving force for mass transfer of carbon monoxide and hydrogen to the liquid phase.
The net result is that conversion efficiencies, especially for hydrogen, in a deep, bubble column reactor are often low. Increasing the depth to provide a longer contact time provides ever diminishing benefits, increases the risk of carbon monoxide inhibition and thus is not a solution by itself to obtain sought high bioconversion efficiencies.
Recycling the off-gases from the top of a deep, bubble column reactor is possible as these gases contain unreacted carbon monoxide and hydrogen. However, as the conversion efficiency of hydrogen and carbon monoxide increases, the off-gases have such a reduced concentration of hydrogen and carbon monoxide, that recycling is impractical and may lead to inefficiencies due to a dilution of the mole fractions of carbon monoxide and hydrogen in the gas feed to the aqueous menstruum.
Processes are therefore sought to capture the benefits provided by a bubble column fermentation system and to be able to enhance the conversion of hydrogen and carbon monoxide without undue risk of carbon monoxide inhibition.